(1) Field of the Invention
The present invention generally relates to a solenoid valve device, and more particularly to a solenoid valve device of an antilock brake system for automotive vehicle.
(2) Description of the Related Art
An antilock brake system (ABS) of an automotive vehicle is known. The antilock brake system promotes the stability and controllability of the vehicle during operation when the vehicle is braked. In a proposed solenoid valve device of the antilock brake system, a plurality of solenoid valves are arranged on a block, and the block includes oil passages. A master cylinder is connected to four wheel cylinders through the oil passages. The wheel cylinders are arranged to carry out the braking operations of the four wheels of the vehicle by means for producing the oil pressure. The oil pressure in each of the wheel cylinders is controlled by actuating the solenoid valves.
The ABS actuator is usually composed of eight solenoid valves, and the braking operation of each of the four wheels of the vehicle is controlled by actuating two solenoid valves among the eight solenoid valves. The oil pressure is produced at the master cylinder owing to the force of operating a brake pedal, and the master cylinder supplies the oil pressure to the wheel cylinders via the respective oil passages, in order to perform the braking operation of the four wheels. One of the two solenoid valves mentioned above is a pressure holding valve for holding the oil pressure at the wheel cylinder, and the other is a pressure reducing valve for reducing the oil pressure at the wheel cylinder. Thus, the four pressure holding valves and the four pressure reducing valves are arranged in the ABS actuator.
There is the need for a smaller solenoid valve device since the mounting space of the automotive vehicle is limited. A large mounting space becomes necessary if various solenoid valves are separately arranged within the automotive vehicle.
Japanese Laid-Open Patent Application No. 2-256979 discloses a proposed solenoid valve device in which a plurality of solenoid valves are arranged on a single block. The purpose of the proposed solenoid valve device is to satisfy the need for a smaller solenoid valve device. Each of the plurality of solenoid valves is composed of a solenoid coil and a valve member free to slide along the coil axis under the influence of the magnetic field. A control circuit which supplies electric current to the respective solenoid valves is provided in the proposed device. The magnetic field within each of the solenoid valves is established by a flow of the electric current from the control circuit, so as to control the solenoid valves, independently of each other, in accordance with the locking conditions of the respective wheels of the automotive vehicle.
In the proposed solenoid valve device mentioned above, the plurality of solenoid valves and the control circuit for controlling the solenoid valves are arranged to satisfy the need for a smaller device. However, when one solenoid valve is operated by supplying electric current to the coil of that solenoid valve, the magnetic flux of the magnetic field produced within that solenoid valve may leak to the other solenoid valves, adjacent to that solenoid valve, arranged on the block. Thus, the other adjacent solenoid valves on the block may be erroneously operated by the leaking magnetic flux. It is difficult for the proposed solenoid valve device to prevent the magnetic flux of the operated solenoid valve from leaking to the adjacent solenoid valves.